THE CEPHALOPODA 337 
slight development of the pro-ostracum, and the large size of the 
phragmocone. The group is probably derived from the straight 
Tetrabranchia, such as JBactrites. In the living genera, with the 
exception of Spirula, the shell is a chitinous gladius. 
FamIty 1. BELEMNOTEUTHIDAE, Zittel. An extinct family in which 
the shell has a well-developed phragmocone and the rostrum is reduced 
to a calcareous envelope surrounding it; the siphuncular necks are 
directed backwards, as in the Nautiloidea ; there were ten equal arms 
provided with hooks. |Genera— Phragmoteuthis, Mojsisovics ; Trias. 
Belemnoteuthis, Pearce ; Jurassic and Cretaceous (Fig. 262, A). Acantho- 
teuthis, Wagner and Miinster; Jurassic. Faminy 2, AULACOCERATIDAE, 
Fischer. An extinct family in which the shell is formed of a phragmmocone 
ae 
* Frye. 295. 
Spirula. A, dorsal aspect; B, ventral aspect. a, arms, e, eyes; fi, fins; fu, funnel; pa, 
mantle; po, posterior fossa; sh, shell; te, tentacular arms; t.d, terminal pallial disc. (After 
Chun.) 
with widely distant septa ; the siphuncular necks are directed anteriorly ; 
the rostrum is well developed and claviform. Genera—Aulacoceras, Hauer ; 
Trias. Atractites, Giimbel; Trias and Jurassic. Xviphoteuthis, Huxley ; 
Lias. Faminy 3. BrLemNiTIpAr, de Blainville. An extinct family 
with a short phragmocone provided with a ventral siphuncle and 
prolonged dorsally into a long pro-ostracum; the rostrum highly 
developed and cylindrical. Genera— Belemnites, Lister ; 350 species 
from the Jurassic and Cretaceous. Diploconus, Zittel ; Upper Jurassic. 
Famity 4. Betoprertpar. Rostrum and phragmocone well developed ; 
the phragmocone often curved ; the initial chamber small. Genera— 
Beloptera, de Blainville; Eocene. Bayanoteuthis, Munier-Chalmas ; 
Eocene. Spirulirostra, d’Orbigny ; Miocene (Fig. 262, C). Faminy 5. 
SPIRULIDAE, d’Orbigny. The two dorsal and ventral sides of the aboral 
22 
